The field of the invention relates to structural fatigue detection and more particularly to detecting crack length of a structure during out-of-plane excitation by a shaker.
The U.S. Air Force, other government agencies and the automotive industry are interested in high cycle structural fatigue materials to support aircraft and vehicle sustainment and payload capability. High-cycle fatigue is a critical requirement in the development of new materials and structures for airframe and automotive components that will operate under extreme conditions. High-cycle fatigue testing is accomplished by exposing the material or structure to a preselected or known number of vibration cycles and then detecting the corresponding fatigue by determining the onset of a crack. The exposure to vibration cycles is intended to closely simulate an operational environment.
Currently, an electrodynamic shaker employing out-of-plane excitation is used to fatigue aircraft or automotive materials and structures. Out-of-plane excitation refers to exciting a structure perpendicular to the surface area of the structure. During such fatigue testing, the occurrence of a crack is the only criterion for the generation of stress-versus-cycles-to-failure curves for various structural materials. That is, current fatigue testing techniques do not include the capability to detect the length or degree of a crack or failurexe2x80x94merely the existence of a crack or failure. Research in the field of fatigue testing includes investigating methods of determining crack length versus cycles that are similar to the techniques used for in-plane tension/compression fatigue tests. For example, test plates are fatigued using in-plane excitation to generate cracks and traducers and recorders are used to count test cycles.
Problems with known fatigue detection methods include: the inability to detect the onset of a crack during normal operation of the structure, detected cracks grow very rapidly; the vibration of the structure does not allow test personnel to view the crack length versus time or test cycles; the crack is usually extremely thin. These problems make detecting the cracks using conventional video capture techniques extremely difficult. To overcome these limitations, test personnel must stop the excitation (shaker) in order for the crack length to be measured. This can seriously impact the crack-size-versus-cycle curve since the input force must be increased and decreased, meaning that the force input is not kept constant. Techniques for tension/compression crack-size determination require use of gages and clamps which can also affect the dynamic response of the structure under test. Also, attached gages may not survive the vibration test.
The U.S. Air Force is currently attempting to increase the structural life of aging aircraft by applying durability patches after initiation of a crack. Durability patches are made of structural adhesive, viscoelastic material and fiberglass or composite repair material and operate by reducing stress levels at crack tips. Increased knowledge of crack initiation and growth will help verify the utility of such durability patches. Additionally, the ability to determine where a crack will likely form prior to failure will allow preventative maintenance to supplant catastrophic failure.
One significant aspect of structural failures is that a substantial change in the surface temperature occurs as dynamic-fatigue increase. Heretofore, this significant aspect of structural failures has not been successfully employed in detecting cracks or crack length. One possible prior art method of detecting cracks and measuring crack lengths involves the use of a thermography system based on an infrared camera. Drawbacks to this technique include the substantial cost of such systems (well over $200,000 for adequate range and sensitivity) and the large background signal due to ambient heat sources.
There is currently no known valid and accurate way to visually measure crack onset and crack length of a structure during in-laboratory investigation or during a flight test. An accurate, reliable technique to determine crack growth rates is needed in the art. The method and device of the invention provides a technique to accomplish accurate crack onset and growth measurement in structures in an inexpensive, accurate, and non-contacting manner.
The present invention provides a method and device for visually detecting crack onset and length of a temperature sensitive paint coated test structure during electrodynamic excitation or during flight testing. The method and device of the invention capitalizes on the increased surface temperature changes of the test structure as structural fatigue increases. Test structure surface temperature changes manifest as fluorescence intensity changes in the temperature sensitive paint and are video recorded. Improvements over conventional structural fatigue systems include the ability to record fatigue prior to crack formation, the ability to detect crack length and the ability to detect fatigue during flight testing.
It is therefore an object of the invention to measure crack length while vibrating a structure on an electrodynamic shaker.
It is another object of the invention to measure crack onset while flight testing a structure.
It is another object of the invention to measure crack length of a structure without making contact with the structure.
It is another object of the invention to measure fatigue of a structure being tested on an electrodynamic shaker without stopping the structural exciter.
It is another object of the invention to measure the crack length on a structure being tested relative to the number of excitation cycles applied to the structure.
It is another object of the invention to determine structural crack formation earlier in the failure cycle sequence than conventional methods.
These and other objects of the invention are described in the description, claims and accompanying drawings and are achieved by a structural fatigue investigating visual crack length measurement device comprising:
a test structure coating and test structure fatiguing temperature and intensity responsive temperature sensitive paint;
a test structure excitation signal generator;
a test structure excitation signal recorder; and
a thermal sensitivity variation, temperature sensitive paint coated test structure viewing video system, video recorded variations in said temperature sensitive paint corresponding to test structure fatigue.